In many parts of the world, potable water is in short supply. Many processes have been proposed and used to remedy this shortcoming by producing potable water from brackish water, brine, seawater and the like. Such processes are typically used to recover distillate from water which contains at least one dissolved solid material. Typically the water fed to such processes is filtered to remove particulates before charging to the process with the dissolved solids being removed by distillation to produce a distillate and a brine stream which is carrying the dissolved solids.
Widely used processes of this type include multi-effect distillation processes, multi-stage flash processes, reverse osmosis and the like, as disclosed in “Understand Thermal Desalination,” Ettouney, Hisham M., El Dessouky, Hisham T., and Alatiqi, Imad, Chemical Engineering Progress, September, 1999. These distillation processes have long been used to produce distillate from feed water streams containing dissolved materials. These processes have been used for the production of desalinated (distillate) water from seawater, salt water, brackish water and the like. Basically, such thermal distillation processes use one or a series of vessels (effects or stages) and use the principles of evaporation, flashing and condensation at reduced pressure in the various effects.
Processes using a single vessel are known but more commonly multi-vessel systems are used commercially to produce distillate. Single vessel systems are more widely used on ships and the like. Various designs have been used for the heat exchangers in such distillation processes, such as horizontal tubes with a falling water film on the outside, vertical tubes with a falling water film on the inside or plates with falling water films and the like. Also various methods for adding feed water to multi-vessel distillation systems are known and used widely.
The thermal efficiency of these processes depends in part upon the number of vessels which may vary from one to 35 or more vessels. These vessels require a heat source to supply an initial source of heat which is carried through the system to produce additional distillate in each of the vessels. This heat has been supplied for thermal distillation processes by a variety of systems, such as the use of steam, mechanical vapor compression, thermal vapor compression, absorption vapor compression, adsorption vapor compression, and the like. Such processes are well known to those skilled in the art for use in multi-effect distillation processes. The commercial utility of such processes frequently depends upon the ability to obtain heat from a low cost source to enable the evaporation and flashing of the water in the various vessels to produce the distillate economically.
Compressor intercoolers are commonly used in gas compressors, air compressors and compressor sections of turbine systems to reduce the energy required while compressing a fluid. An intercooler is a heat exchanger that cools a hot compressed fluid from a preceding compressor stage before entering a subsequent stage of compression, thereby decreasing the work necessary in the subsequent compression. It is desirable to provide a low temperature cooling fluid to the intercooler to optimize the temperature reduction of the compressed fluid entering the intercooler and thus minimize the work of compression in the subsequent compressor stages as appropriate. Generally the intercooler may be cooled by an air or water stream with the heat supplied by the intercooler simply rejected to the environment.
When an intercooler is employed with a turbine system, the stream compressed is an air stream. Hot air from the low-pressure compressor enters the intercooler where the air is cooled by heat exchange with a cooler stream. The cooled air is then returned to the high-pressure compressor for further compression. The final compressed air stream is then mixed with a fuel stream and the mixture is combusted and expanded in a turboexpander which produces shaft power which may be used to drive the compressor and to produce electricity or the like. The hot exhaust gas stream produced in the turbine may be used in a heat recovery system to produce a heated stream, such as steam or the like, which is usable to drive a turboexpander or the like to produce additional shaft energy which may be used to drive an electric generator, produce additional power or the like. Such systems are well known to those skilled in the art and do not require extensive discussion.
Typically the energy recovered by a heat recovery section is limited by the sulfur content of the fuel which is burned in the combustion turbine. Use of low sulfur fuels, such as natural gas, can maximize the amount of heat recovered in the heat recovery section allowing lower exhaust gas temperatures than the exhaust gas temperatures of high sulfur fuels, such as fuel oil. This is due to the exhaust gases from a combustion turbine which burn low sulfur fuels having a lower sulfuric acid dew point. Heat recovery sections typically employ recirculation systems or the like to maintain an exhaust gas temperature above the water or the sulfuric acid dew point in order to minimize corrosion of the lower temperature parts in the heat recovery section.